Stokes parameters are parameters that represent a state of polarization. When measuring Stokes parameters, incident signal light is split into four rays using splitting means such as a beam splitter, a half-mirror, or a filter; herein, each of the signal light rays is given a different polarization and phase by means of a phase element, a polarizing element or the like, then the optical components of each of the split signal light rays are subjected to photoelectric conversion by a light-receiving element, and operations are performed on the electrical components obtained by photoelectric conversion, to obtain the Stokes parameters (see Japanese Patent Application Publication No. 6-18332). There are also methods in which, instead of a splitting means comprising a beam splitter or the like, there is used a quadrangular pyramidal prism or the like for concurrent four-beam splitting (see Japanese Patent Application Publication No. 2004-93549).